1. Field of the Invention
This invention relates generally to a method of making an elastomer-metal composite article having an elastomeric member in compression, more particularly to an elastomer-metal bonded article with an expanded cellular elastomeric member comprising a platy filler in compression between two rigid surfaces.
2. Description of the Prior Art
Articles combining rubber or elastomeric members with metal or other rigid, structural members find many uses. Of particular interest are articles subject to dynamic or repeated stresses. Examples include various vibration control devices, such as vibration isolating mounts for vibrating equipment, vibration dampers, isolators, couplers, shock absorbers, and the like. In general, the rubber member provides a vibration damping or isolating function and the structural member provides a means for attaching the rubber or the article to other equipment, such as to the source of the vibration, or to a stationary location or support, and/or to freely vibrating masses. The elastomeric member may hold together two or more structural members and function as a connecting spring or have a structural role. Herein, “structural member” will not refer to the elastomeric member. Structural members are typically of metal but could be of other relatively rigid materials. Herein, such rubber-metal parts will be referred to generally as composite articles. It should be understood that composite articles may encompass any combination of one or more elastomeric or rubber members and one or more rigid, structural members of metal, structural composite, or other relatively rigid material.
In composite articles subject to dynamic stresses, it is known that maintaining a degree of compressive stress sufficient to prevent tensile deformation of the rubber is beneficial for extending the fatigue life of the rubber member. If the rubber is subject to repeated tensile deformation, then cracks will have a tendency to grow more rapidly. If the rubber is first subject to compression, such crack growth tendencies can be minimized. By compressive stress or load is meant a stress or force tending to reduce a linear dimension of a rubber member. Compression or a state of compression may be characterized by the deflection or linear deformation exhibited under a compressive stress or load. ASTM D395 or D575 provides a useful way to characterize compression based on the percent change in a linear dimension, i.e. a compressive deformation or deflection. Likewise, tension or elongation may be characterized by an increase in linear dimension of a rubber member under a tensile force or stress as defined in ASTM D1566.
One way to form a composite article with a rubber member in compression is to forcefully insert or press fit a cured or vulcanized rubber member into a rigidly defined gap between two rigid members that is smaller than the thickness of the rubber member. If it is desired to adhesively bond vulcanized rubber and metal members together, one or both of the surfaces of rubber and/or metal may be treated with an appropriate adhesive which may be activated by heat after assembly. Generally, a state of compression in the rubber member may be maintained by this so-called “post-vulcanization bonding” process. However, multiple steps are required, including first forming and vulcanizing the rubber member, then cleaning the rigid member, applying adhesive, then assembling the rubber and rigid members, and finally curing the adhesive. Moreover, compression set or relaxation effects are detrimental to the state of compression.
On the other hand, if adhesion is obtained by vulcanizing the rubber member in place against the metal surfaces, a so-called “vulcanization bonding” process, thermal shrinkage after cooling after vulcanization is likely to remove any compression created during assembly and put the rubber member into a state of tension. While post-vulcanization bonding is generally superior for maintaining rubber members in compression, vulcanization bonding may generally produce more robust adhesion between rubber and metal. Vulcanization bonding can also be accomplished with fewer process steps than post-vulcanization bonding, including for example the elimination of the separate adhesive, but the inability to maintain the rubber member in compression can severely limit the durability of the assembled part in use.
U.S. Pat. No. 7,291,241 describes a two-step cure method whereby a partially cured rubber member may be assembled under compression and then bonded in the absence of a separate adhesive. By controlling the degree of partial cure in the first step, the amount of compression retained in the final part can be significant. However, the degree of compression attainable or retained may still not be sufficient to assure long life under dynamic load conditions, and the cure state may be difficult to control consistently. Compression set and/or relaxation effects may reduce or eliminate any degree of compression achieved initially. Improved or alternate methods of forming a rubber-metal or other elastomeric composite part having an elastomer under compression may be desirable, especially for dynamic applications.
Various articles and processes have been suggested utilizing foam or cellular rubber instead of solid rubber. U.S. Pat. No. 6,077,135 suggests using “rubber, foam, epoxy, or some other flexible material” compressed and/or bonded between a torsional damper's outer hub and ring. U.S. Pat. No. 6,026,709 suggests using a compressible material, if necessary, of porous plastic material, a foam material, or rubber sponge in a crankshaft damper as a torsional elastic coupling element. Why compressibility (presumably volumetric or bulk compressibility) would be necessary is not mentioned, but may be to make the coupling or rubber spring so flexible that the first fundamental frequency is very low. In general, rubber sponge is much softer than the solid material equivalent, and the cells are “open” or inter-connected.
U.S. Pat. No. 2,271,498 describes a conventional process for making closed-cell cellular rubber wherein the rubber is heated and partially vulcanized in a closed mold to a point at which gas is generated, then releasing the rubber from the confining mold to permit expand up to three to five times the original volume to fill a second larger mold cavity where complete vulcanization also occurs. U.S. Pat. No. 3,855,378 describes a process for preparing closed-cell expanded rubber in which vulcanizable rubber with a blowing agent is partially cured at a temperature lower than that at which substantial decomposition of the blowing agent occurs and subsequently expanding and completing the cure of the partially cured rubber. The examples therein indicate that significant blowing occurs upon partially curing, with linear expansions of about 12 to 75%. After blowing and final curing, the rubber has expanded about eight-fold in volume. Pat. No. GB 1,115,573 suggests no blowing takes place during vulcanization when using a high temperature blowing agent with activation temperature at least 20° C. above the vulcanization temperature. The final cellular articles are expanded about 100% in at least one direction, or about eight-fold in volume. Pat. No. GB 829,912 adapts the blowing process to run continuously for making expanded rubber extrusions. Pat. No. GB 737,485 utilizes an expandable mold so that partial vulcanization and expansion in one direction with secondary vulcanization can be carried out without transferring the material between two molds. Pat. No. GB 720,549 discloses a process for bonding sponge or cellular rubber to a metal insert or object using an adhesive and/or unvulcanized rubber film layer on the metal surface, followed by conventional expanding and vulcanization bonding in a suitable mold.